Typically, the technologies used to produce electronic circuits and electrode parts in particular have been pattern-screen printing, photo-patterning, or etching copper conductor foils via a photo-resist marking process. Among the three processing methods, only the screen printing process is an additive process. However, it is not digitally controlled. Since the trend in the electronics industry is to make smaller and cheaper electronic devices which require higher resolution and finer conductor lines for performance, it has become necessary to develop conductor materials and processes to achieve these goals.
The use of ink jet printing of conductive materials to substrates for electronic circuit production is both a digital and additive process which provides a less expensive, faster, more environmentally conscious and more flexible method of electronic circuit production. Piezo ink jet technology is the current focus because of its Drop-On-Demand capability.
Typically, piezo ink jet technology can only print liquids with a viscosity of under 20 m·Pas·s measured at the moment of jetting. Such a low viscosity makes it difficult to make a stable, high density dispersion, such as a dispersion containing conventional-size silver particles. This is especially true when the metal particles are larger than a few hundred nanometers in diameter. Another difficulty when a conductor composition has low visicosity and contains a low content of conductor materials is to obtain narrow-in-width yet still thickly printed conductor lines. Thus, the resulting ink jet-printed, thin conductor lines on a plain substrate surface tend to have low conductivity. Nanometer-sized (or nano-size) and colloidal conductor particles may help increase the loading of conductor materials in a stable, low viscosity ink composition. This in turn helps to produce thick ink jet printed conductor lines. However, conductor lines of the prior art made of nano-size particles tend to disconnect or break down during the high temperature firing that is necessary for many ceramic substrate-based applications.
U.S. Pat. No. 5,132,248 to Drummond et al., discloses a process for forming a pattern on a substrate by deposition of a material, consisting of: (a) depositing a suspension of colloidal particles of the material in a solvent on to a substrate by ink jet printing; (b) evaporating the solvent, the material remaining on the substrate; (c) laser annealing the deposited material to the substrate, the pattern being defined by the path of the laser beam; and (d) removing excess material not annealed by the laser beam.
EP 0 989 570 A1 to Nakao et al., teaches an ink or an electronic component comprising water or organic solvent, and a resin dispersed in said water or organic solvent, by 1 wt. % or more to 80 wt. % or less, at viscosity of 2 mPas·s or less. EP 0 989 570 A1 further teaches a method for manufacturing an electronic component comprising the steps of: repeating a plurality of times a process of forming a specified ink pattern on a ceramic green sheet by an ink jet method using an ink prepared by dispersing metal powder with particle size of 0.001 μm or more to 10 μm or less, in at least water or organic solvent, by 1 wt. % or more to 80 wt. % or less, at viscosity of 2 poise or less; laminating a plurality of the ceramic green sheets forming this ink pattern to form a raw laminated body of ceramic; cutting to specified shape and baking, and forming an external electrode.
JP Kokai Patent Application No. P2000-327964A to Nakao teaches an electronic part electrode ink having a viscosity of 2 P or below, formed by dispersing metal powder of particle diameter 10 μm or less in water or organic solvent at a concentration of 1-80 wt. %, and having a precipitation of 10 mm or less in 10 min or 20 mm or less in 100 min.
The present inventor desired compositions that can be applied by ink jet printing technology onto various substrates while at the same time these compositions are characterized as stable dispersions that contains a large amount of solids (for example, silver metal powder) with a viscosity less than 20 m·Pas·s at the moment of jetting.